Several months ago we purchased a nice, low-mileage (22,000) Sonata
GLS with 2.5 V6 and 4-speed automatic from a local Hyundai dealer.
About two days after we purchased the car, the transmission failed
right in the dealer's lot when we took it in for a minor service! They
handled the problem without quibble, installing a new transmission and
giving us a loaner during the service. (Fantastic warranty!)

However, since the transmission has been replaced I've noticed a
high-pitched whine that seems to be coming from the drivetrain when
under load. It is most noticable at part throttle around 25 to 40 mph.
I don't think it's an engine or belt noise as the pitch varies with
road speed, not engine speed; higher road speed yields a
higher-pitched whine. (Does not change when shifting gears.) Also when
you let up off the gas there is a delay of a second or two before the
whine goes away. Doesn't sound like tires as far as I can tell. I'm
not very familiar with these cars mechanically, but on other vehicles
I would suspect a touch of pinion or torque converter whine.

When the car was brought back for an oil change I reported the noise
to the service manager, who later said the tech couldn't reproduce it.
(Admittedly it's not a very loud noise, but it is noticable and other
Hyundais I've since driven in don't have it.) They asked that I take a
drive with a technician to demo the noise, which I'll do at the next
oil change.

In the meantime I thought I'd ask here: anyone have an inkling of what
could be causing this? Obviously if there is a serious problem
developing I'd want it taken care of while the car is still under
warranty!